Organic electroluminescent (EL) devices, also referred to as organic light-emitting devices, contain spaced electrodes separated by an organic light-emitting structure (also referred to as an organic EL medium) which emits light in response to the application of an electrical potential difference across the electrodes.
In recent years, preferred organic EL devices have been constructed by cmploying thin film deposition techniques. Using an anode as the device support, the organic EL medium is deposited as one or a combination of thin films followed by the deposition of a cathode, also formed as a thin film deposition. Thus, starting with the anode structure, it is possible to form the entire active structure of an organic EL device by thin film deposition techniques. Examples of organic EL, devices containing organic EL medium and cathode constructions formed by thin film deposition arc provided by commonly-assigned U.S. Pat. Nos. 4,356,429; 4,539,507; 4,720,452; and 4,769,292.
While the art has encountered little difficulty in constructing fully acceptable stable anodes for the organic EL devices, cathode construction has been a matter of extended investigation. In selecting a cathode material, a balance must be struck between materials having the highest electron-injecting efficiencies and those having the highest levels of stability. The highest electron-injecting efficiencies are obtained with alkali metals, which are too unstable for convenient use, while materials having the highest stabilities show limited electron-injection efficiencies.
To provide an organic EL device with a cathode having acceptable electron-injection efficiency and acceptable environmental stability, commonly-assigned U.S. Pat. No. 4,885,211 teaches the formation of a cathode of a combination of metals, with at least 50 percent (atomic basis) of the cathode being accounted for by a metal having a work function of less than 4.0 eV. Commonly-assigned U.S. Pat. No. 5,047,687 teaches the use of a cathode containing a plurality of metals, at least one of which is a low work function metal other than an alkali metal.
Commonly-assigned U.S. Pat. No. 5,677,572 discloses a bilayer electrode on an N-type semiconductor. This electrodes includes a non-conducting layer contacting the semiconductor and a conductive layer contacting the non-conducting layer. The materials and the thickness of the non-conducting layer are selected so that the bilayer forms a low resistance contact to the semiconductor, and provides a bilayer having acceptable stability against atmospheric corrosion. More particularly, U.S. Pat. No. 5,677,572 teaches that the non-conducting layer of the bilayer electrode can be selected from the group of alkali or alkaline earth fluorides or oxides, and the conductive layer can be selected from the group of elemental metals, metal alloys, nitrides, sulfides, and oxides. The non-conducting layer has a thickness in a range of 0.3-5.0 nanometer (nm).
U.S. Pat. No. 5,429,884 discloses an organic electroluminescent element having a transparent anode, a hole-carrying layer, a light-emitting layer, and an alloy cathode consisting of a first metal lithium and a stabilizing second metal aluminum, in which the concentration of the first metal lithium in the alloy region is controlled within a predetermined thickness from the interface between the alloy cathode and the organic layer. In a second embodiment, the first metal of an alloy cathode is stronthium and a stabilizing second metal is magnesium.
As evidenced by the aforementioned disclosures, substantial efforts have been made to provide improved electron-injection into an N-type semiconductor layer from an electrode, and to enhance the stability of such electrodes in organic EL devices.
In contrast to the disclosures in the above cited commonly-assigned U.S. Pat. Nos. 4,885,211; 5,047,687; and 5,677,572 and of Namiki et al U.S. Pat. No. 5,429,884 appear to be directed to an electron-injecting electrode in which a first electron-injecting component of the electrode is in contact with the N-type semiconductor, and a second component of the electrode serves to either stabilize the first component (such as in the case of the U.S. Pat. No. 5,429,884 device) or to provide an electrically conductive feature to the non-conducting first component (such as in the U.S. Pat. No. 5,677,572 bilayer electrode). Stated differently, the cathodes described by U.S. Pat. Nos. 4,885,211 and 5,047,687 comprise a mixture or an alloy of a low work function metal and a higher work function metal, wherein the mixture or alloy extends uniformly throughout the cathode from a surface of an N-type organic semiconductor to an upper surface of the cathode, while the non-conducting layer of the U.S. Pat. No. 5,677,5721 bilayer electrode contacting the N-type semiconductor extends between 0.3-5.0 nm in thickness therefrom, and the alloy region of the U.S. Pat. No. 5,429,884 cathode extends from above 0 nm to 150 nm in thickness from an organic light-emitting layer.
Ultra-thin layers in contact with a semiconductor surface are the subject of an ongoing debate and discussion among experts in this field to provide a better understanding of the electron-injection processes from interfacial layers which may be as thin as a few atomic dimensions.